This invention relates generally to a pressure vessel assembly, and more particularly to a pressure vessel assembly for housing electronic components in an underwater environment.
Underwater communication systems provide signal transmission between, or to, land-based positions. A typical system generally comprises a cable for signal transmission and one or more housings containing electrical components which are spaced along the cable between the land-based positions. The signal cable has a core which may include electrical conductors, fiber optic cable, or other signal transmission elements surrounded by a protective jacket. Metal strength members are disposed about the core of the cable between the cable core and the outer surface of the jacket. The strength members bear the tensile and compressive loads placed on the cable while in operation. The cable may carry various signals, including low voltage signals such as information and data signals, higher voltage signals for providing electrical power, or other types of signals.
The housings for the electronic components are referred to as pressure vessels. A pressure vessel is typically a cylindrical tube with open ends capped by circular bulkheads. The signal cable is terminated adjacent to each of the bulkheads in a termination assembly. The termination assemblies are secured to the bulkheads and provide mechanical continuity between the cable ends and the pressure vessel while relieving the stress on the signal transmission elements of the cable. The signal transmission elements pass into the pressure vessel through seals in the bulkheads for connection to the electronic components in the pressure vessel.
The pressure vessel assembly must protect the electronic components, signal cable and their connections from exposure to water at depths of up to 20,000 feet and pressures of up to 10,000 pounds per square inch for periods of up to 25 years. This harsh environment contributes to problems related to the performance reliability and product life of pressure vessel assemblies.
Conventional solutions to the problems of designing reliable and durable pressure vessel assemblies are plagued by high cost. The present practice is to use pressure vessels formed from beryllium-copper or titanium with polyethylene or gland cable seals and polyethylene-overmolded cable termination assemblies. Beryllium-copper or titanium is used for the pressure vessel because of their excellent resistance to corrosion in underwater applications. However, this material is very expensive, and machining is difficult. The polyehtylene or gland cable seals are expensive, consist of numerous parts, and are difficult and time-consuming to install. In addition to the seals, a water block of some sort must be used to prevent water ingress in the event of a cable cut.
The termination assemblies are overmolded with polyethylene to prevent water from accessing the internal portions of the signal cable and pressure vessel. In the overmolding process, high density polyethylene is molded around the cable termination assembly thereby sealing the areas between the outer surface of the cable jacket and the termination assembly. In some cases, portions of the cable and the pressure vessel are also overmolded with polyethylene. However, polyethylene overmolding is not cost effective in most applications because the required molding equipment is expensive and the process time consuming thereby restricting production rates. Moreover, the overmolding is a difficult process, requiring a high operator skill level and has yield and reliability problems.
For the foregoing reasons, there is a need for a reliable, long life, low cost pressure vessel assembly for housing electrical components in underwater communication systems. The new pressure vessel assembly must be capable of withstanding deep underwater pressures for many years. Seals for the passage of the signal cable transmission elements into the pressure vessel should be easy to install and effectively prevent moisture penetration. The termination assembly sealing process should be fast and simple to perform, requiring minimal operator skill level. Moreover, the components for sealing the cable termination assembly should be adaptable to seal various cable termination assembly types.
Therefore, it is an object of the present invention to provide a low cost underwater pressure vessel assembly which is readily and economically produced for use in underwater communications systems.
A further object of the present invention is to provide moisture protection to the electronic components in the pressure vessel. A related object is to provide a simple, effective seal for the signal transmission elements passage into the pressure vessel.
A still further object of the present invention is to provide a seal and method for readily and simply sealing a cable termination assembly for use in connecting signal cable to the pressure vessel. The termination assembly seal and method should allow for adaptability to various cable types.
Another object of the present invention is to provide a pressure vessel assembly which is reliable and durable enough for an extended useful life submerged in the underwater environment.
According to the present invention, a pressure vessel is provided for housing electronic components in an underwater environment and permitting connection of the components to signal transmission elements of a signal cable. The pressure vessel comprises a hollow steel shell defining an interior chamber adapted to house the electronic components. A layer of thermal-sprayed aluminum covers the shell. The shell has an opening adapted to pass the signal transmission elements into the interior chamber. A seal adapted to sealingly surround the transmission elements is disposed in the opening in the shell so that an outer peripheral surface of the seal contacts an inner peripheral surface of the opening in the shell for preventing moisture penetration into the interior chamber. The seal may be formed of epoxy and the outer peripheral surface have a plurality of compressible o-rings disposed in axially-spaced circumferential grooves for contacting the inner peripheral surface of the opening in the shell.
Also according to the present invention, an underwater telecommunication system is provided comprising electronic components and a hollow steel shell for housing the electronic components. The shell is covered by a layer of thermal-sprayed aluminum. A signal cable is mechanically connected to the shell in watertight relation. The signal cable includes at least one transmission element and the shell has an opening for passing the transmission element into the interior chamber for connecting the transmission element to the electronic components for signal transmission. A seal surrounding the transmission element is disposed in the opening in the shell for preventing moisture penetration into the shell.
According to another aspect of the present invention, a cable seal is provided for sealing the passage of a cable into a cable-receiving structure. The seal comprises a first plurality of o-rings disposed along a length of the cable adjacent the cable-receiving structure and a first length of heat-shrinkable tube which fits over the cable and o-rings for compressive engagement with the cable and o-rings when the first tube is heated to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed over the first heat-shrinkable tube adjacent the first plurality of o-rings. A second length of heat-shrinkable tube fits over the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure for compressive engagement with the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure when heated to prevent moisture penetration between an inner surface of the second tube and an outer surface of the cable-receiving structure and between the inner surface of the second tube and the outer surface of the cable.
According to a still further aspect of the present invention, a sealed cable end assembly comprises a structure having a through passage for receiving a cable end and means for preventing relative axial movement of the structure and cable. A first plurality of o-rings is disposed along a length of the cable adjacent the cable-receiving structure. A first length of heat-shrinkable tube is positioned around the cable and o-rings for compressive engagement with the cable and o-rings when the first tube is heated to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed over the first heat-shrinkable tube, the second plurality of o-rings positioned adjacent the first plurality of o-rings. A second length of heat-shrinkable tube fits over the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure for compressive engagement with the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure when heated to prevent moisture penetration between an inner surface of the second tube and an outer surface of the cable-receiving structure and between the inner surface of the second tube and the outer surface of the cable.
Also according to the present invention, a method is provided for sealing a cable termination including a cable end positioned in an opening of a cable-receiving structure for preventing relative axial movement of the cable end and structure. The sealing method comprises disposing a first plurality of o-rings along a length of the cable adjacent the cable-receiving structure and positioning a first length of heat-shrinkable tube around the cable and o-rings. The first heat-shrinkable tube is heated causing the tube to compressively engage the cable and o-rings to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed around the first heat-shrinkable tube adjacent the first plurality of o-rings and a second length of heat-shrinkable tube is positioned around the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure. The second heat-shrinkable tube is then heated causing the tube to compressively engage the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure to prevent moisture penetration between an inner surface of the second tube and an outer surface of the portion of the structure and between a portion of the inner surface of the second tube and the outer surface of the cable.
The pressure vessel assembly of the present invention provides a low cost, structurally robust, reliable, and durable device for use in underwater communication systems. The thermal-sprayed aluminum coating of the steel pressure vessel provides a housing for the electronic components of the system which is resistant to corrosion, particularly galvanic corrosion in seawater. The two-layers of heat-shrinkable tubes over sets of o-rings function as multiple redundant seals for the cable termination assembly. The components of the pressure vessel assembly of the present invention are low cost and require minimal assembly expertise and time.